The present invention is directed generally to the field of optical trapping techniques. More particularly, the present invention relates to techniques for manipulating particles and fluids using the torques and forces exerted by optical vortex traps.
Optical tweezers use forces exerted by intensity gradients in strongly focused beams of light to trap and selectively move microscopic volumes of matter. Capable of applying precisely controlled forces to particles ranging in size from several to tens of nanometers to tens of micrometers, single optical tweezers have been adopted widely in biological and physical research. Holographic optical tweezers expand upon these capabilities by creating large numbers of optical traps in arbitrary three-dimensional configurations using a phase-modulating diffractive optical element (DOE) to craft the necessary intensity profile. Originally demonstrated with microfabricated diffractive optical elements, holographic optical tweezers have been implemented by encoding computer-designed patterns of phase modulation into the orientation of liquid crystal domains in spatial light modulators. Projecting a sequence of trapping patterns with a spatial light modulator dynamically reconfigures the traps.
Each photon absorbed by a trapped particle transfers its momentum to the particle and tends to displace it from the trap. If the trapping beam is circularly polarized, then each absorbed photon also transfers one quantum, n, of angular momentum to the absorbed particle. The transferred angular momentum causes the trapped particle to rotate in place at a frequency set by the balance between the photon absorption rate and viscous drag in the fluid medium. Laguerre-Gaussian modes of light can carry angular momentum in addition to that due to polarization. Bringing such a Laguerre-Gaussian beam to a diffraction-limited focus creates a type of optical trap known as an optical vortex. In additional to carrying angular momentum, optical vortices have other properties useful for assembling and driving micromachines, for pumping and mixing fluids, for sorting and mixing particles, and for actuating microelectromechanical systems.
The present invention describes a practical and general implementation of dynamic holographic optical tweezers capable of producing hundreds and even thousands of independent traps for all manner of materials and applications.
Unlike conventional micromanipulators, dynamic holographic optical tweezers are highly reconfigurable, operate noninvasively in both open and sealed environments, and can be coupled with computer vision technology to create fully automated systems. A single apparatus thus can be adapted to a wide range of applications without modification. Dynamic holographic optical tweezers have widespread applications in biotechnology. The availability of many independent optical manipulators offers opportunities for deeply parallel high throughput screening, surgical modifications of single cells, and fabrication of wide-spectrum sensor arrays. In materials science, the ability to organize disparate materials into three-dimensional structures with length scales ranging from tens of nanometers to hundreds of micrometers constitutes an entirely new category of fabrication processes with immediate applications to photonics and fabrication of functional nanocomposite materials.
The applications described herein take advantage of a related method for transferring angular momentum to optically trapped particles. The technique uses computer-generated diffractive optical elements to convert a single beam into multiple traps, which in turn are used to form one or more optical vortices. The present invention involves combining the optical vortex technique with the holographic optical tweezer technique to create multiple optical vortices in arbitrary configurations. The present invention also involves employing the rotation induced in trapped particles by optical vortices to assemble clusters of particles into functional micromachines, to drive previously assembled micromachines, to pump fluids through microfluidics channels, to control flows of fluids through microfluidics channels, to mix fluids within microfluidics channels, to transport particles, to sort particles and to perform other related manipulations and transformations on matter over length scales ranging from roughly 5-10 nm to roughly 100 xcexcm. Several applications and related extensions derive from the properties of optical vortices.
Other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.